1. Field of the Invention
The present invention relates to a symmetric optical modulator with low driving voltage, and more particularly, to a symmetric optical modulator with low driving voltage, wherein polarization of any one of branched waveguides formed on a substrate is inverted, and the two branched waveguides are simultaneously controlled by a center electrode formed on a top portion thereof, ensuring a low voltage driving and featuring no signal distortion generated by chirp.
2. Description of the Related Art
Generally, optical modulators refer to optical devices, wherein a radio frequency (RF) is applied to an electrode located on a top surface of a LiNbO3 substrate so that optical characteristics of optical waveguides can be changed, whereby change of outputted light is so induced as to identify itself with a shape of the input RF signal.
The optical modulator is largely classified into two kinds of modulators according to crystal orientation of an LiNbO3 substrate and mutual locations of optical waveguides, each kind being referred to as a z-cut optical modulator and an x-cut optical modulator.
FIGS. 1 and 2 are plan and sectional views schematically showing a structure of a general z-cut optical modulator. The z-cut optical modulator (100) comprises an LiNbO3 substrate (101), an optical waveguide (102) formed by diffusing Ti onto a top surface of the LiNbO3 substrate (101), a buffer layer (103) coated onto an entire top surface of the LiNbO3 substrate (101),and three electrodes (104, 105, 106) formed on a top surface of the buffer layer (103) to transmit electric signals to the waveguide (102).
The electrode (105) positioned at the center of the three electrodes (104, 105, 106) is a positive electrode, and the other electrodes (104, 106) are ground electrodes.
A concept of operating the z-cut optical modulator thus constructed will now be described. Laser light is inputted to an optical waveguide, branched off from branched optical waveguides, and combined into an output optical waveguide and then outputted.
If a voltage difference between the center electrode (104) and the outer electrodes (106, 104) is produced by an RF signal applied to the center electrode (105), a phase velocity of the laser light transmitted to an optical waveguide located underneath the center electrode (104) is decreased whereas a phase velocity of the laser light transmitted to an optical waveguide located underneath the right electrode (106) is increased. As a result, when the two laser lights reach the output optical waveguide, the lights show a phase difference of 180° and then are cancelled out, so that an optical signal of ‘0’ is outputted from the output optical waveguide.
On the other hand, if no voltage difference between the center electrode (104) and the outer electrodes (106,105) is produced, the laser lights transmitted to the optical waveguides are reinforced, so that an optical signal of ‘1’ is outputted from the output optical waveguide.
Meanwhile, to maximize efficiency of an optical modulator, it is essential to fabricate electrodes with minimized RF loss while meeting phase velocity matching between light and RF, 50Ω impedance matching of electrodes, and the like.
To meet all the above characteristics at the same time, a buffer layer thicker than is necessary is required. Further, this increase in the thickness of the buffer layer reduces the intensity of electric fields applied to optical waveguides, resulting in necessity of much higher driving voltage.
There have been conducted many studies on minimizing the increase of the driving voltage due to the aforementioned causes.
FIGS. 3 and 4 are schematic plan and sectional views showing a structure of a general z-cut optical modulator with low driving voltage, respectively. The z-cut modulator (200) with low driving voltage comprises a LiNbO3 substrate (201) provided with two protruding regions (201a, 201b) on a top surface thereof, an optical waveguide (202) formed by diffusing Ti into each of the protruding regions (201a, 201b) on the top surface of the LiNbO3 substrate (201), a buffer layer (203) applied on an entire surface of the LiNbO3 substrate (201), and three electrodes (204, 205, 206) formed on a top surface of the buffer layer (203) to transmit electric signals to the waveguide (202).
The z-cut optical modulator (200) with low driving voltage ensures phase velocity matching with light and impedance matching even in the buffer layer (203), which is thinner than that of the conventional structures, by etching portions of the substrate (201) existing between the electrodes (204, 205, 206).
Accordingly, the driving voltage can be greatly reduced. Such a structure is well known in the prior art (U.S. Pat. No. 5,790,719), so that a detailed description thereof will be omitted herein.
However, since the intensity of electric fields applied to the optical waveguide located underneath the center electrode (204) is several times larger than that of the optical waveguide located underneath the outer electrode (206) due to the difference in locations of the two waveguides, intensity variations and phase shifts of the outputted light are produced.
A signal distortion caused by the phase shift is called chirp, which acts as a major constraint to the long distance transmission as amount of transmission is further increased. As a result, in the case of a large-capacity optical communication of which transmission rate is above 40 Gbps, an x-cut modulator with almost no chirps is preferably used.
FIGS. 5 and 6 are plan and sectional views schematically showing a structure of the general x-cut optical modulator, respectively. The x-cut modulator (300) comprises an LiNbO3 substrate (301), an optical waveguide (302) formed by diffusing Ti into a top surface of the LiNbO3 substrate (301), a buffer layer (303) applied onto an entire surface of the LiNbO3 substrate (301), and three electrodes (304, 305, 306) formed on a top surface of the buffer layer (303) to transmit electric signals to regions where the optical waveguide (302) is not formed.
Unlike the z-cut modulator, the optical waveguide can be located between electrodes in such an x-cut optical modulator. Therefore, chirp can be minimized because two optical waveguides are symmetrically located.
However, there is a problem in that the driving voltage is in high since the intensity of electric fields applied to the optical waveguides in the x-cut modulator is relatively small as compared with that of the z-cut modulator. There is another problem in that this modulator is not greatly different from the modulator shown in FIGS. 1 and 2, in view of their thickness of the buffer layer.
Accordingly, there are urgent and earnest needs for a novel electrode structure capable of maximizing the intensity of electric fields applied to an optical waveguide while maintaining a symmetric characteristic of an x-cut modulator.